[Bug]: macOS backend reports wrong button for button_release_event #31899

Bug summary

Run the attached code with the qtagg as well as the macosx backends.

While the mouse cursor is over the figure, perform the following:

1. Click mouse. Release mouse
2. Hold Option: Click mouse. Release mouse.
3. Hold Control: Click mouse. Release mouse.

Code for reproduction

import matplotlib.pyplot as plt

def on_button_press(event):
    print(f'button press: {event.button}')
def on_button_release(event):
    print(f'button release: {event.button}')

fig = plt.figure()
fig.canvas.mpl_connect('button_press_event',   on_button_press)
fig.canvas.mpl_connect('button_release_event', on_button_release)

plt.show()

Actual outcome

qtagg:

button press: 1
button release: 1
button press: 1
button release: 1
button press: 3
button release: 3

macosx:

button press: 1
button release: 1
button press: 2
button release: 1
button press: 3
button release: 1

Expected outcome

Based on these code comments in _macosx.m:

if (modifier & NSEventModifierFlagControl)
    /* emulate a right-button click */
    button = 3;
else if (modifier & NSEventModifierFlagOption)
    /* emulate a middle-button click */
    button = 2;

I would expect:

button press: 1
button release: 1
button press: 2
button release: 2
button press: 3
button release: 3

However, if alt+click should not result in "middle click", I would expect macosx to match qtagg.

Additional information

No response

Operating system

macOS 14.8.5

Matplotlib Version

dev

Matplotlib Backend

qtagg, macosx

Python version

3.14.5

Jupyter version

No response

Installation

None